To ensure that drainage of wastewater occurs in the forward direction from upstream to downstream, float-type backflow prevention valves for prevention of backflow have conventionally been installed in wastewater drainage systems that cause wastewater generated within buildings or on the premises of factories or the like to be discharged into rivers, oceans, public sewer facilities, and so forth. Float-type backflow prevention valves are exemplified by Japanese Utility Model Application Publication Kokai No. H6[1994]-67588.
A float-type backflow prevention valve might typically be provided with an annular valve seat by way of which an upstream side and a downstream side of a wastewater drainage system may be placed in mutual communication, a float that is disposed below the annular valve seat, a plurality of guide members that guide the float in a vertical direction, and so forth. The float rises and falls to open and close the annular valve seat in accompaniment to water level of the wastewater. Because water level is low during normal operation, this causes the float to fall onto (rest on) a base below the annular valve seat, opening the valve at the annular valve seat. Drainage of wastewater is thus able to occur in the forward direction from upstream to downstream as a result of passage through the annular valve seat. On the other hand, because water level rises when there is occurrence of backflow, this causes the float to rise, closing the valve at the annular valve seat. As a result, backflow into (flooding of) locations upstream of the annular valve seat is prevented.
Furthermore, where a plurality of buildings are, for example, connected to a single wastewater drainage system, it may be that wastewater generated at each of the respective buildings is handled by the single wastewater drainage system. Where this is the case, a plurality of float-type backflow prevention valves might be installed at the wastewater drainage system to prevent backflow to the respective buildings.